The impartial reason wh we lost is that we came out flat, and missed a lot of shots we usually made, while the hawks came out hot and made some shots they usually miss. Both teams mostly defended well, and the shots missed that usually go in made the difference.
The emotional knee-jerk reaction (as in the one I feel in my soul), is that the celtics didn't get tough until the second half and didn't box out, while the refs completely bailed on a few calls.